The present invention relates to a new process for preparing rubber reinforced styrenic resins, in particular ASA resins, which are copolymers of styrene, unsaturated nitriles, acrylates and conjugated dienes.
It is well known that styrene homopolymers and copolymers of styrene and unsaturated nitriles have a poor impact strength. In order to increase this impact strength, rubber is added to these styrene polymers. The principal methods which have been used are: blending of a rubber latex with a polystyrene latex, milling of a dry rubber with a dry polystyrene, or polymerization of styrene in the presence of an unsaturated rubber. This latter method is the most widely employed, as the products obtained are more stable and have superior properties when compared with products prepared with the other methods, based upon the same amount of contained rubber. The generally used rubbery polymers are conjugated diene polymers and/or acrylic rubbers. The resulting styrenic resins are usually called impact polystyrene, ABS resins or ASA resins, according to their composition.
One of the usual techniques for preparing rubber-reinforced styrenic resins consists in preparing the rubbery compound, in a first-step, by emulsion polymerization of a monomer, such as for instance, a diolefin or a mixture of an aliphatic ester of acrylic acid with a low amount of a difunctional monomer, to obtain a latex containing cross-linked particles of rubber. In a second step, chains of homo-or copolymers of styrene are grafted onto the particles of the pre-formed rubber in order to obtain a sufficient degree of dispersion together with a good adhesion of the rubber phase in the continuous phase comprised of the styrene homo- or copolymers. In a third step, the grafted latex is coagulated and dried.
However, this method presents several drawbacks. For instance, the end product always contains a substantial amount of emulsifier which has a detrimental effect on the properties of the desired product. Another method comprising a mass prepolymerization of styrene in the presence of a rubber followed by a suspension polymerization gives products having high impact resistance and also better properties than similar products obtained by an all emulsion process or by an all suspension process. With this two-step method, the monomer to be grafted on the rubber must be a solvent for this rubber. At the beginning, the mixture of rubber and monomer, for instance styrene, forms a homogeneous phase, which is subjected to a partial mass polymerization step, under vigorous agitation. A polystyrene phase separates from the solution, and the polymerization system thus comprises two phases: a continuous phase consisting of rubber dissolved in styrene and a discontinuous phase consisting of polystyrene in styrene. Concomitant with the formation of polystyrene, graft copolymers are also produced. After reaching a certain point in conversion of the monomer, a phase inversion occurs and the polymerization is continued by suspension polymerization. The rubber-in-styrene phase is now the discontinuous phase suspended in a matrix of polystyrene dissolved in styrene. Vigorous stirring is required during the mass prepolymerization in order to control the sizes of the rubber particles which must be comprised between about 1 and 5 82 m. Other factors also play a role in this process, and in some cases the viscosity of the system after the mass prepolymerization step is so high that it is not possible to obtain a suspension of the copolymer. This process must be carefully carried out, and it therefore has some limitations.
In order to obviate the drawbacks of the hereinabove described processes, it has already been proposed to graft butadiene on a previously irradiated styrenic compound-acrylonitrile compound copolymer, by dispersing the copolymer in a solution of hexane and butadiene in order to induce an absorption of butadiene by the copolymer. However, this process results in a low degree of grafting, and it is necessary to remove the unreacted butadiene. Moreover, the impact strength of these copolymers is lower than that generally required for such compositions.
It has also been proposed to use sequentially produced polymers as additives to other rigid thermoplastics. The process for manufacturing these additives comprises at least three stages. It comprises first forming a non-rubbery, hard polymer core by emulsion polymerization of a first monomer charge, particularly styrene. In a second stage, a second monomer charge containing butadiene and/or an alkyl acrylate is added to the hard polymer core and is emulsion polymerized with formation of a rubbery polymer stage. Thereafter, a third monomer charge containing an alkyl methacrylate is added to the rubbery polymer and is emulsion polymerized to form a third stage polymer substantially encapsulating the polymer produced in steps 1 and 2. In this composite, the hard phase polymeric methacrylate comprises a cover or layer for the inner mass and imparts compatibility to the composite with the rigid thermoplastic polymer. The inner mass comprises a rubber polymer which forms a continuous phase and which is grafted onto the styrene polymer. The sequentially produced polymer is an elastomeric material which imparts improved impact resistance and clarity to vinyl halide polymer compositions and other rigid thermoplastic polymers. See, for example, U.S. Pat. Nos. 3,793,402 and 3,971,835. It acts therefore as impact improver and is quite different from a rubber-modified polystyrene, wherein the rubber acts as impact improver for the polystyrene.
The exact mechanism by which discrete particles of rubber increase the impact strength of glassy polystyrene is still controversial. However, it has been shown that the rubber is particularly effective when it is present during the polymerization of styrene. Grafting of styrene to rubber takes place and occlusion of polystyrene extends the volume fraction of the dispersed rubber phase.